U.S. Patent Application Publication No. 2004/134560 A1 (corresponding to Japanese Patent Application Publication No. 2004-37779) discloses an electrophotographic-type image forming apparatus that is provided with a developer cartridge detachably mounted on the apparatus main body. The inner space of the case of the developer cartridge is generally divided by a partition wall into a toner accommodating chamber and a developing chamber. In the toner accommodating chamber, toner serving as developer is contained and an agitator for agitating the toner is rotatably provided. A pair of shaft bearing portions that support the shaft portion of the agitator are disposed on both sides of the toner accommodating chamber. A toner filling opening for filling the toner is formed on one side wall of the toner accommodating chamber at the opposite side to the developing chamber with respect to the shaft bearing portions. At the lower portion of the developing chamber, a developing roller that visualizes an electrostatic latent image on a photosensitive member with toner and a supply roller that supplies the toner to the developing roller are provided so as to be rotatable in pressure contact with each other. The bottom wall of the developing chamber is formed in a shape that encircles the developing roller and supply roller so that the toner is smoothly supplied thereto. A small clearance is formed between the bottom wall and outer circumference of the both rollers.